pet sitter vs custodian

pet sitter and custodian both are nouns.

Word	Noun	Adjective	Verb	Adverb
pet sitter	Yes	No	No	No
custodian	Yes	No	No	No

As nouns, custodian is a hypernym of pet sitter; that is, custodian is a word with a broader meaning than pet sitter:

pet sitter: someone left in charge of pets while their owners are away from home
custodian: one having charge of buildings or grounds or animals

Other hypernyms of pet sitter include keeper, steward.

pet sitter (noun)	custodian (noun)
someone left in charge of pets while their owners are away from home	one having charge of buildings or grounds or animals